Field of the Invention
This invention relates to magnetic systems such as ion implanters that scan ion beams of atoms and molecules comprised of light and heavy elements, and in particular to an ion beam scanner for such an ion implanter.
Background of the Invention
There are numerous industrial and scientific applications that require surfaces to be uniformly irradiated by ion beams. For example, modification of semiconductors such as silicon wafers is often implemented by irradiating the wafer with a beam of specific ions or molecules of a specific energy. Because the physical size of the wafer or substrate (e.g., about 200 mm-300 mm in diameter or more) is larger than the cross-sectional area of the irradiating beam (e.g., about 50 mm in diameter or less), the required uniform irradiance is commonly achieved by scanning the beam across the wafer or scanning the wafer through the beam, or a combination of these techniques.
It is distinctly advantageous to have a high beam scan rate over the substrate for a number of reasons: the irradiance uniformity is more immune to changes in the ion beam flux; a higher wafer throughput is possible at low dose levels; and for high dose applications degradation from local surface charging, thermal pulsing, and local particle-induced phenomena such as sputtering and radiation damage are greatly reduced.
Scanning techniques based only upon reciprocating mechanical motion are very limited in speed. Motion of the wafer on an arc through the beam greatly improves the scan speed but requires many wafers or substrates to be simultaneously mounted on a rotating carousel in order to obtain efficient utilization of the beam.
In a common variation, a time varying electric field is used to scan the beam back and forth in one direction, while the wafer is reciprocated in another direction. In this hybrid type of implanter the beam current and hence rate at which wafers can be processed is severely limited by the space-charge forces which act in the region of the time-varying deflecting electric fields. These forces cause the ions in the beam to diverge outward, producing an unmanageably large beam envelope. Such a space-charge limitation also occurs in implanters that use time-varying electric fields to scan the beam in two directions. Also, electric field scanning becomes more difficult to implement at high ion energies because of the large electric fields required.
As a result, magnetic scanning techniques have been developed and are used extensively in the manufacture of semiconductor devices and also for exfoliating thin films of substrates such as silicon, sapphire and silicon carbide. In ion implanters employing magnetic scanning techniques, ions enter a scanning magnet in a beam from an ion source. The beam exits the scanning magnet as a divergent fan beam. This fan beam is then formed into a (more or less) parallel ribbon using a collimator magnet downstream of the scanning magnet, such as is described in U.S. Pat. No. 5,438,203 and U.S. Pat. No. 5,311,028. The resultant ribbon beam is then directed toward a wafer or other target substrate to be implanted, where it arrives at the surface of the target along a constant preselected direction irrespective of the position of the beam on the target substrate.
An early example of a scanning magnet for ion implantation is described in U.S. Pat. No. 5,311,028. The arrangement described therein employs a scanning magnet that permits the scanning of high perveance, heavy ion beams at frequencies of up to 1 kHz.
One of the problems of scanning magnets is that the electron gyro radius r of neutralizing electrons in the ion beam increases as the magnetic field strength B of the scanning magnet decreases. As the magnetic field strength approaches zero, electron orbits describe an outward spiraling envelope, thus reducing the electron density in the region of the ion beam. The consequence of this is that the space charge neutralization in the beam changes as the scanning magnetic field used to scan the ion beam passes through or approaches zero. This generally results in a beam size fluctuation during the zero field crossing leading in turn to a degraded uniformity of irradiation of the wafer.
In the aforementioned U.S. Pat. No. 5,438,203, a solution to the problem of beam size fluctuation during zero field crossing is proposed. A magnetic deflection system is described with a magnetic scanning structure. The magnetic scanning structure has laminated poles separated by insulating layers, and ac coils which are energized by a scanning current source. In use, an excitation current is applied to the ac coils which results in a unipolar scanning magnetic field above a predetermined value, in the gap between the pole faces. That predetermined value is sufficiently greater than zero that the B field does not approach a zero field crossing. In that manner, the beam size does not fluctuate at the wafer or substrate.
Nevertheless, the solution to the problem of non-uniform implantation due to zero field crossing, as proposed in the above mentioned U.S. Pat. No. 5,438,203, places increased demands on the power consumed by the ion implanter, because the reactive power of a unipolar scanning magnetic field as disclosed therein, is significantly greater than the reactive power of a bipolar scanning magnetic field. Such issues are exacerbated as commercial endeavors to increase the diameter of the wafers to be implanted up to 450 mm or more are undertaken. There is also a desire to reduce the capital cost of an ion implanter.
U.S. Pat. No. 5,481,116 also addresses the problem of zero field crossing beam size fluctuation. Here, a scanner magnet is formed of a magnetic structure with poles having faces to form a gap through which the ion beam passes. Ac coils are associated with the poles. A current is applied to the ac coils to produce a bipolar scanning field. Dc current carrying coils are disposed adjacent the gap and produce a dc magnetic field component which is orthogonal to the bipolar (ac) magnetic field component in the gap. The interaction of the ion beam with the ac and dc magnetic fields means that the beam never experiences a zero field crossing and hence the beam emittance remains stable.
In addressing the zero field crossing problem, the arrangement of U.S. Pat. No. 5,481,116 thus does not suffer from the increased reactive power issues of U.S. Pat. No. 5,438,203. It does however suffer from different drawbacks. The arrangement of U.S. Pat. No. 5,481,116 uses a well-defined pole boundary within the gap through which the ion beam passes. Both the ac coils and the dc coils have a bobbin type construction. This results in a relatively non uniform dc magnetic field which causes variations in the transverse deflection of the ion beam. The consequence of this is that there is an ion optical deterioration of the beam size at the downstream wafer to be implanted.
The above referenced U.S. Pat. No. 5,438,203 proposes a sector collimator magnet with pole edge contours that are fourth order polynomials. Using terms up to fourth order permits an increased deflection of ions, to reduce the number of neutral molecules arriving at the wafer to be implanted, whilst the beam control requirements (parallelization, dimensional and angular constraints, etc.) continue to be met.